Micro ATX or ATX

I have been looking around and found the Biostar M7NCG400 to be a good option for in car use as I dont want to head the route of EPIA, especially has it has capable on board graphics, negating the need for a card. As far as I know it also supports XP-M which is very beneificial.

What I would like to ask, is whether there is a difference in power usage between a Micro ATX board and an ATX board, if I was to use an XP-M in both cases. Also, would turning on board devices off in the BIOS save on power usage?

I am hoping to use an 1800 XP-M, Micro ATX boatd, 80Gb Desktop HD, and 2 Ram sticks, and the LinITX 8" TS. Would this run under an OPUS 150 or less?